1. Field of the Invention
The present invention relates to a data retaining device, and more particularly to a data retaining device for a camera.
2. Description of the Prior Art
In some cameras, calculation control has hitherto been performed electronically, with a battery being used as the power source. In this type of calculation control, various data are input to memories. However, conventional calculation control encounters the problem that, when the battery of the camera is replaced, the power supply to the memories is interrupted, and, as a result, information stored therein is lost. A known countermeasure that is taken against this problem involves a method which comprises providing an auxiliary battery as a back-up so as to keep the memories constantly supplied with power from the auxiliary power source. However, this is not a perfect method of retaining data because auxiliary batteries have limited lives and are expensive. In order to overcome these defects, a proposal has recently been made in Japanese Patent Laid-Open No. 61731/1985 concerning a data storage apparatus which is provided with a switch which may be turned off when a lock-button provided for opening and closing the cover of the battery receptacle is brought to its lock-releasing position, and in which data stored in the main memory is transferred in response to a signal indicating the off-condition of the switch to a non-volatile storage element such as an E.sup.2 PROM into or from which data can be electrically written or read, or erased.
However, with the data storage apparatus disclosed in Japanese Patent Laid-Open No. 61731/1985, it is necessary to transfer data within a time interval of only about 25 ms (milliseconds) between the time at which the cover for the battery receptacle is released from its locking engagement and the time at which the cover is opened, thereby limiting the amount of data that can be transferred.